1. Field of the Invention
The present invention relates generally to radio receivers and, more particularly, is directed to electronic apparatus having a synthesizer-type receiver which is preferably combined with a cassette type tape player and/or a compact disk (CD) player in a unitary assembly.
2. Description of the Prior Art
It is known to provide, in a unitary assembly, an AM/FM radio receiver with a cassette tape player and/or a CD player, and in which the AM/FM receiver, the cassette tape player and the CD player are independently controlled by respective micro-computers.
It is also known to employ an AM/FM receiver of the synthesizer-type in such unitary assembly. For example, as shown in FIG. 1, a known electronic apparatus comprises, in a unitary assembly, an AM/FM receiving section 1 of the synthesizer-type, a CD player section 2, a cassette tape player or deck section 3, a main or master micro-computer 10, slave micro-computers 11, 12 and 13 for controlling the AM/FM receiver section 1, the CD player section 2 and the cassette tape player section 3, respectively and an additional slave micro-computer 14 for processing control signals supplied from or to a remote control receiver 15 and transmitted from or to a remote commander (not shown). The AM/FM receiver section 1 of the synthesizer-type conventionally includes a phase locked loop (PLL) in which a frequency-dividing ratio signal from the micro-computer 11 determines the dividing ratio of a variable frequency-dividing circuit (not shown) in the PLL and thereby determines the receiving frequency of the receiver section 1.
The micro-computers 11-14 may each be constituted by a 4-bit single chip-type micro-computer, and are all controlled or commanded by the main computer 10. When the operations of the receiver section 1, the CD player section 2, the tape cassette player section 3 and the remote control receiver 15 are controlled by the micro-computers 11, 12, 13 and 14, respectively, and such micro-computers 11-14 are controlled by the main micro-computer 10, the software programs in the micro-computers 11-14 can be easily developed so as to enable the sections 1, 2 and 3 to perform numerous operations and functions. In a practical example of the prior art arrangement illustrated in FIG. 1, the main micro-computer 10 is a micro-processor obtainable from the NEC Corporation under the designation D 753284GC, the micro-computer 11 is a micro-processor also obtainable from the NEC Corporation under the designation D 1715G, the micro-computer 12 is obtainable from Sony Corporation under the designation CXP 5078, the micro-computer 13 is also obtainable from Sony Corporation under the designation CXP 5024, and the micro-computer 14 is obtainable from Mitsubishi Electric Corporation under the designation M 50920.
It will be appreciated that a clock signal is required for operation of each of the micro-computers 10-14. If the clock signal CK for the main micro-computer 10 has a relatively high clock frequency fc, for example, of 4.2 MHz, then as shown in FIG. 2A, higher harmonic components Sh of the clock signal CK are distributed within the FM broadcasting frequency band of 76 to 108 MHz. As a result, when the receiver section 1 is operative to receive a FM signal broadcast at a frequency that is equal to one of the higher harmonics Sh of the clock frequency fc, the reception of such broadcast signal is disturbed.
In order to avoid the above problem, it has been suggested, for example, as shown on FIG. 2B, that the frequency fc of the clock signal CK be decreased to approximately the lowest frequency at which the main computer 10 can be operated, for example, to a frequency of 400 kHz, so that the order of the higher harmonic components Sh of the clock signal CK distributed within the FM broadcasting band is substantially higher than the order of the higher harmonic components within the FM broadcasting band in FIG. 2A. By reason of the foregoing, when the frequency fc of the clock signal CK of the main micro-computer 10 is decreased to about 400 kHz, the level of the higher harmonic components Sh of such clock signal within the FM broadcasting band is so small as to be negligible. More specifically, in the case illustrated in FIG. 2A, the higher harmonic components Sh of the clock signal CK distributed in the FM broadcasting band when the clock frequency fc is 4.2 MHz are in the range from the 19th order to the 24th order, and at least some of the levels thereof are of sufficient magnitude to produce the undesired disturbance of the FM reception. On the other hand, in the case illustrated in FIG. 2B, and in which the frequency fc of the clock signal CK is decreased to 400 kHz, the higher harmonic components Sh of the clock signal CK distributed within the FM broadcasting band are from the 190th order to the 270th order. When the higher harmonic components Sh of the clock signal CK within the FM broadcasting band range from the 190th order to the 270th order, the levels of such higher harmonic components are, as shown, so small as to be satisfactorily neglected and to ensure that reception of the FM broadcast signal will be undisturbed. When the frequency of the clock signal CK for the main computer 10 is reduced as described above for avoiding disturbance of the reception of the FM broadcast signal by the higher harmonic components of such clock signal, the frequencies of the clock signals employed in the micro-computers 11-14 will be similarly decreased for the same reason.
Of course, if low clock frequencies are used in the micro-computers 10-14, as described above, the processing speeds of the micro-computers 10-14 are correspondingly lowered. Since the micro-computers 11-14 are employed for independently controlling the respective sections 1, 2 and 3 and remote control receiver 15, such computers do not need to operate at high speed. Therefore, the clock frequencies of the micro-computers 11-14 can be reduced, as described above, to ensure that the higher harmonic components of the clock signals employed in the microcomputers 11-14 have negligible levels.
However, the main micro-computer 10 exerts overall system control of the electronic apparatus and the micro-computers 11-14 are, in turn, operated under the control of the main micro-computer 10 so that, if the frequency fc of the clock signal CK of the main micro-computer 10 is lowered, for example, to 400 kHz, as described above, the correspondingly lowered processing speed of the main micro-computer 10 reduces the range of operations of the receiver section 1, the CD player section 2, the tape cassette player section 3 and the remote control receiver 15 can be controlled by the main computer. For example, in the case where audio levels are displayed, the reduced processing speed of the main micro-computer 10 will introduce a time delay in the audio level display. Thus, the audio level indication will not correspond to real time conditions.